Olefinic products, referred to generally herein as C.sub.4 olefins or as olefinic products, are commonly produced in catalytic dehydrogenation processes using propane, butane, pentane, or mixed paraffins as starting materials. Such processes include the CATOFIN.TM. and CATADIENE.TM. processes developed commercially by Air Products and Chemicals, Inc. of Allentown, Pa, the assignee of the present application. Product streams from these dehydrogenation processes usually also include lighter or higher boiling point gases than the desired olefinic product.
Typically, the olefinic product has been separated from the lighter gases in such streams, in the past, by a product separation sequence which included pressurization of the olefinic-rich stream, absorption of the lighter gases from that stream (in an absorption medium such as heavy oil) and then the stripping of the lighter gases from the absorption medium by distillation. Because absorption took place in this process in the vapor phase, entrained liquid was typically removed during and after compression by simply permitting such entrained liquid to collect and condense as the vapors passes through a drum or small tank, sometimes referred to as a "flash drum" or "knockout drum." This combination of absorber/stripper and distillation column for stabilizing (or "deethanizing" or "depropanizing" as it is sometimes referred to) the product stream is effective to recover a high proportion of the available olefinic product in the feed stream. Cryogenic recovery has also been used, to accomplish the same result, in place of absorption/stripping.
However, the cost effectiveness of these stabilizing processes is greatly impaired by the required size and energy demands of the absorber/stripper (or cyrogenic system) and the volume of material recycled to the process.
It has now been found that the depropanizer vapor overhead stream, which has heretofore been recycled into the compression section of the process, may be more economically separated and recovered by a process of condensation and fractionation. Such a process eliminates the need to recycle vapor for compression in the product compression stage, thus permitting both energy and capital cost savings.